Many communications systems employ communication protocols in which data to be communicated is converted to a system that includes in-phase (I) and quadrature-phase (Q) components. The in-phase and the quadrature-phase components are shifted in phase by an angular amount, such as, for example, 90 degrees. The data signal is impressed on the in-phase and the quadrature-phase signal components by a transmitter and recovered by a receiver. One example of a communication system that employs in-phase and quadrature-phase components is what is referred to as a multiple input multiple output (MIMO) communication system that uses orthogonal frequency division multiplexing (OFDM) in a multiple-antenna arrangement and that complies with communication standard IEEE 802.11n.
In any communication system that uses in-phase and quadrature-phase signal components, the quality of the signal transmission is dependent upon the relationship between the in-phase and the quadrature-phase components. If the in-phase and the quadrature-phase components are not properly matched, also referred to as “I/Q mismatch,” degradation of the communication signal occurs. There are typically three types of I/Q mismatch. These are gain imbalance, quadrature error and I/Q offset. Gain imbalance is caused by a difference in gain between the in-phase component and the quadrature-phase component. Quadrature error is caused by an error in the phase rotation between the between the in-phase component and the quadrature-phase component. I/Q offset is caused by the leakage of a direct current (DC) signal at the receiver input.
I/Q mismatch occurs because of non-ideal modulator and demodulator elements in which the upconverters and attenuators of the I and Q channels are mismatched. I/Q mismatch results in serious performance degradation because it causes inter-carrier interference. A MIMO system is more likely to be influenced by I/Q mismatch than a single input single output (SISO) communication system because there are multiple modulators and demodulators, each of which can introduce I/Q mismatch. Further, the I/Q mismatch may result in inter-channel interference.
The measurement of I/Q mismatch for OFDM and MIMO OFDM systems is challenging in that the I/Q mismatch will result in large channel estimation error if the I/Q mismatch impairment is not correctly compensated. Further, the accurate measurement of I/Q mismatch requires decoupling the influence of I/Q mismatch on the channel estimation. This is especially true for a MIMO communication system under a fading channel situation.
Prior attempts of I/Q mismatch compensation fail to address each of the I/Q mismatch parameters, and further, fail to address I/Q mismatch compensation under a MIMO fading channel situation. Prior attempts at I/Q mismatch measurement and compensation only considered the measurement of the ratio of gain imbalance to quadrature error. Other prior attempts at I/Q mismatch measurement and compensation provided measurement methods for gain imbalance and quadrature error for a SISO communication system without considering the influence of channel fading. Furthermore, these prior attempts all require a specially designed test signal.
Therefore, it would be desirable to have a way to measure and compensate I/Q mismatch, including measuring gain imbalance, quadrature error and I/Q offset under a MIMO fading channel situation without using specially designed signals.